1. Field of the Invention
The present invention relates to a method and apparatus for controlling power to an electrical load. More particularly, the present invention relates to a method and apparatus for controlling power to a high power heater element in a fuser of an imaging device to provide improved warm-up and temperature control characteristics.
2. Description of Related Prior Art
In printing, the amount of time it takes for the first page of a print job to be printed and to reach the printer's output bin is known as first copy time, and is an important feature to users of the printer. In conventional electrophotographic printers, the controlling factor for first copy time typically has been the amount of time it takes to warm up a cold fuser to a target temperature for performing a fusing operation.
To optimize the first copy time, the fuser must be heated as fast as possible. In addition, it is necessary to maintain the temperature of the fuser within a narrow temperature window close to a predetermined target temperature for a given mode of operation of the fuser. These requirements impose conflicting design constraints on a heater element incorporated within a heated fuser roll. For example, it is desirable to have a relatively high power heater element to provide a fast temperature ramp up when initially heating the fuser. On the other hand, when controlling power to such a high power heater element, it is difficult to operate within a narrow temperature window, particularly when small, controlled temperature corrections are required to maintain a target temperature.
A further limitation on the operation of the heater element relates to noise reduction requirements imposed in Europe on all electrical and electronic equipment, known as the “harmonic” requirement IEC 61000-3-2, and the “flicker” requirement IEC 61000-3-3. When power is first applied to the heater element for the fuser, such as a 750 W tungsten-filament lamp or other high wattage lamp, there is typically a large inrush current that primarily produces harmonic noise and an instantaneous voltage drop that can affect other electrical equipment connected to the same or a nearby electrical branch circuit. The effect of the sudden inrush current at the heater element, and associated voltage drop, is readily noticeable as a flicker in the output of fluorescent lights. As the temperature of the heater element rises, its resistance also increases and a larger amount of energy may be applied without the substantial voltage variations experienced during initial warm-up.
One proposed solution to the flicker problem is to control a fuser by using on-off control, i.e., switching power to the fuser heater element on and off, to provide a desired temperature change in the fuser. For example, U.S. Pat. No. 6,097,006 discloses apparatus for increasing the temperature of a fuser in which a switching unit is turned on and off to intermittently disrupt the current supplied to the fuser to warm up the fuser wherein the duration of the “on” relative to the “off” time is selected to provide a desired temperature increase and to control the generation of flicker.
In an alternative approach, U.S. Pat. No. 6,111,230, assigned to the assignee of the present application, discloses a method and apparatus for energizing an electrically driven apparatus that applies power to the apparatus by using phase-angle control. Triggering of the AC power is delayed for each half cycle of the AC current waveform, and in particular is initially delayed by nearly the entire half cycle. The delay time is then decreased at a predetermined rate before triggering each subsequent half cycle until full power is reached.
There is a continuing need to provide a reduced warm-up time for fusers, and in particular to provide a reduced warm-up time for fusers in color laser printers, where the fuser rolls are commonly formed of an aluminum core coated with silicon rubber, having a lower thermal conductivity than the aluminum core, and covered with a fluoropolymer sleeve. The desired reduction in warm-up time may be achieved by providing a high power heater, for example, higher than approximately 800 watts for a single lamp system and 750 watts for a two-lamp system. However, the use of these high power heater elements is dependent on meeting the above-mentioned European harmonic and flicker requirements on electrical equipment. Further, use of such high power heater elements is additionally contingent on providing a control method capable of maintaining the fuser temperature within a narrow range of predetermined target temperatures, such as are defined by target standby and print mode temperatures.